Shortly we'll vanish into the underground at Walthamstow Central. The Victoria line is unusual in two ways; firstly it's entirely underground (the Waterloo and City is also entirely underground, but it doesn't really count -- also it has external access by means of a hole in the ground and cranes at the Waterloo end), and secondly every station except one (Pimlico) is an interchange with existing tube or rail services.
We've already taken advantage of this at Walthamstow, but we're also going to do so at Highbury and Islington where we'll take a short-cut through to Old Street via WAGN's Moorgate service. This was originally a short rail service from Finsbury Park to Moorgate. It became part of the Northern Line (despite sharing no track with it) on rail nationalisation and was an integral part of the
Northern Heightsplan. In the 1970s it was handed over to British Rail, and now commuter trains run from Hartford North and Letchfield through to Moorgate along it.
Critically as far as we're concerned there is one train every 10 minutes during the day, with a "cross platform" connection at Highbury and Islington. At Old Street we'll change to the Northern Line and head towards Edgware (Plan A) or Mill Hill East (Plan B).